1. Field of the Invention
The present invention relates to a wire bonding method for connecting a pad on a chip and an electrode on a substrate through a wire.
2. Description of the Related Art
A wire bonding process for electrically connecting a pad on a chip and an electrode on a substrate such as a printed board and a lead frame through a wire is accomplished in the following manner. First, an electric spark is generated between a lower end of the wire inserted through a capillary tool and a torch so as to form a ball on the lower end of the wire. Then, the capillary tool is lowered so as to press this ball against the pad on the chip, whereby the ball is bonded to the pad. Next, the capillary tool is moved onto the electrode on the substrate so as to press the wire against the electrode, whereby the wire is bonded to the electrode. Subsequently, the capillary tool is lifted and the wire is cut off on a bonding point to the electrode so as to again generate the electric spark between the lower end of the cut wire and the torch, whereby the ball is formed so as to proceed to the subsequent wire bonding operation.
However, due to a removal/attachment of the substrate to be wire-bonded from/to a wire bonding stage or the like, the wire bonding operation is sometimes temporarily stopped. In such a case, the capillary tool is waiting over the stage for a while, and thus the ball formed on the lower end of the wire is hardened during this wait. Then, after the removal/attachment of the substrate or the like is completed, when the wire bonding operation is restarted so as to press this ball against the pad on the chip for the bonding, a problem is caused. That is, since this ball is hardened during the wait, even if the ball is pressed against the pad by a lower surface of the capillary tool, the ball is not sufficiently crushed and thus is prone to a faulty bonding. It is desirable that the ball is crushed by the lower surface of the capillary tool and bonded to the pad on the chip over a wide area.
There is another problem in which, since the ball is brought into contact with air during the above-described wait whereby its surface is oxidized and an oxidized film is thus formed, a material of the ball is deteriorated.
It is therefore an object of the present invention to provide a wire bonding method for solving the above conventional problems and capable of excellently accomplishing a wire bonding immediately after an operation is restarted even after a long waiting time.